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Transport layer

(also Layer 4)

Transport layer definition

A transport layer is the fourth OSI (Open System Interconnection) network layer. Within the framework of protocols and other network components, the transport layer facilitates communication between application processes located on separate hosts. Applications send message fragments to the transport layer, which then sends them onto the network (Layer 3). This layer is the multiplexing component of the network and lets the host send and receive error-corrected data, messages, or packets. Layer 4 transport layers are responsible for error-free data transfer between higher levels. It is the responsibility of the sending end to partition application messages into smaller units called packets before sending them onto the network layer (Layer 3). To reach the application layer, the receiver must piece together the messages from the fragments (Layer 7).

Transport layer functions

  • Connection-oriented communication. Before exchanging data, network devices need a handshake protocol like TCP to establish a secure connection. This method's disadvantage is that each delivered message requires an acknowledgment, which increases the network burden compared to self-error-correcting packets. Repeated queries delay the network when faulty byte streams or datagrams are transmitted.
  • Same order delivery. Assigns a sequence number to each packet to guarantee they are delivered in that order. Even while packet drops and device interruptions are the responsibility of the network layer, the transport layer can reshuffle the packets to correct the order.
  • Data integrity. Protecting data integrity by checksums. With the checksums in place, you can rest assured that your data will arrive in an uncorrupted form regardless of how far it travels.
  • Multiplexing. The transport layer has specific control methods for multiplexing packet streams from unrelated applications or sources over a network. Multiplexing lets computers run numerous internet browsers at once over a network. The OSI service layer handles multiplexing.
  • Flow control. Controlling the rate at which information is transmitted.
  • Traffic control. The transport layer is able to recognize the signs of overloaded nodes and decreased throughput, and take corrective action.
  • Byte orientation. The transport layer can send byte streams instead of packets for applications that prefer them.

Transport layer protocols

  • UDP. Utilized when size and speed outweigh reliability and security. It adds checksum error control, transport-level addresses, and length information to upper-layer data.
  • TCP. To provide a connection-oriented service for data transmission, it defines the phases of connection formation, data transfer, and connection pull-down. TCP is by far the most used one.